3.1Advanced Software Functions

Introduction

As PLC control systems become more complex, the need for real-time

 

processing of analog signals is needed at the I/O level. Current

 

implementations using the 505 controllers utilize analog alarm blocks

 

and/or special function programs within the controller. The SIMATIC

 

505±2557 analog input module from Siemens Energy & Automation, Inc.,

 

can reduce the program complexity and scan time by performing this signal

 

processing in the module.

 

Scaling, alarming, peak/valley hold, digital filtering, and averaging are

 

available on a per-channel basis and are selected through a simple PLC

 

configuration routine. When these advanced functions are enabled, the

 

module logs in as 16X / 16Y / 32WX / 32WY. A jumper on the module selects

 

the standard 16WX login or the high-density advanced function interface.

Overview of the

Each of these functions can be selected on a per-channel basis, and each

Advanced

channel can have any function in any combination, e.g. alarming on a scaled

Functions

value which is digitally filtered and set for peak hold. (See Section 3.4 for

 

timing considerations.)

 

Scaling Each channel can be configured with low and/or high scale value.

 

A flowmeter that outputs 0 mA @ 5 cfm and 20 mA @ 50 cfm would have a

 

low scale of 5 and a high scale of 50. An operator interface attached to the

 

controller could then read the analog values directly in engineering units

 

without having to run a Special Function program to scale the input.

 

Alarming Each channel can be assigned a low and/or high alarm value.

 

No analog alarm blocks are needed in the controller. Alarming occurs

 

real-time as the signal is processed by the module. Two WX words are used

 

to indicate high and low alarm conditions (bit 1 = channel 16, etc.). A third

 

WX word is the logical OR of the high and low alarms.

 

Peak/valley hold The peak or valley of a rapidly changing analog signal

 

has been impossible to detect unless an external circuit was used. The

 

SIMATIC 505±2557 makes possible the detection of a peak or valley and

 

holds that value until reset by the controller. The peak/valley measurement

 

is available to the controller at the same time as the currently measured

 

analog value.

 

Averaging This option is used to ªclean upº a signal that is at a steady

 

state, e.g., a sensor riding on a liquid tank with riplets. The user specifies

 

how many signal scans to average and this value is presented to the

 

controller.

 

Digital filtering This has the effect of a moving average operation

 

(actually it is an Infinite Impulse Response filter), and is useful to smooth

 

out the high frequency noise on a changing analog signal. See Section 3.4.

3-2

Advanced Function Programming

SIMATIC 505±2557 Installation and Operation Guide

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Siemens 505-2557 installation and operation guide Advanced Software Functions, Introduction, Overview